A masking technique has been known in which, in order to form a comfortable environmental space in a worksite or the like, a sound that is felt uncomfortable by the listener is picked up, and another sound having acoustic characteristics (such as frequency characteristics) similar to the sound is output, thereby causing the uncomfortable sound to be hardly heard. For example, Patent Document 1 discloses a technique in which the frequency components of picked-up sounds in the periphery of the listener are analyzed, and a sound that, when mixed with the ambient sound, becomes another sound is produced and then output. The technique of Patent Document 1 can give the listener a comfortable sound which is different from the uncomfortable sound, without reducing the uncomfortable sound, and provide an environmental space which is comfortable to the listener.